Optical writing type liquid crystal light valve and writing apparatus therefor

ABSTRACT

A liquid crystal light valve has a photoconductive layer between a pair of electrodes, and a liquid crystal layer. The resistivity of the photoconductive layer at the portion which is irradiated with light is reduced by partial light irradiation. By applying a voltage between the electrodes in this state, the voltage is applied to the liquid crystal layer in the portion which is irradiated with light and the crystalline structure at this portion is changed, thereby enabling image data writing. The photoconductive layer is a laminate of an amorphous Si film and an inorganic insulating film disposed on the electrode side, which structure suppresses the carrier injection from the electrode to the amorphous Si film. The polarity of a voltage for writing image data into the liquid crystal light valve is inverted for every horizontal scanning operation. That is, writing of black portions on a white background and writing of white portions on a black background are alternately executed. It is therefore possible to write data on one line and erase the data on the next line by a voltage of one polarity. By periodically setting a period in which no voltage is applied during writing, writing error caused by stored charges is prevented.

This application is a divisional of copending application Ser. No.07/822,109, filed on Jan. 17, 1992, now U.S. Pat. No. 281,282 the entirecontents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical writing type liquid crystallight valve which is used for a large-screen high-definition projectiondisplay and the like, a method of driving the same and a writingapparatus used for the same.

2. Description of the Related Art

FIG. 1 is a sectional view of a conventional light valve which isdescribed in, for example, CONFERENCE RECORD OF 1990 INTERNATIONALTOPICAL MEETING ON OPTICAL COMPUTING (1990), 9B3, pp. 17 to 18.

In FIG. 1, the reference numeral 1 represents a glass substrate, 2 atransparent conductive film composed of ITO (indium tin oxide) or thelike, 4 a liquid crystal layer, 5 an orientation film for orientating aliquid crystal, 6 a reading light reflecting plate, and 7 aphotoconductive layer composed of a hydrogenated amorphous silicon filmwhich has a high electrical resistance.

The operation of this light valve will now be described. When data iswritten into a liquid crystal display device, a predetermined bias isfirst applied between an opposing pair of transparent conductive films.In this state, writing light having any given bright and dark spacepattern is caused to enter the photoconductive layer 7. At this time,the photoconductive layer 7 has a high resistance at the points whichare not irradiated with the writing light, while the resistance of thephotoconductive layer 7 is lowered at the points which are irradiatedwith the writing light, due to the photoconductive effect. In this way,the carrier distribution which corresponds to the space pattern of thewriting light is formed on the interface between the photoconductivelayer 7 and the liquid crystal layer 4. In correspondence with thecarrier distribution, a spatial distribution is produced due to thebirefringence or the optical rotational power of the liquid crystal. Forexample, if a ferroelectric liquid crystal is used for the liquidcrystal layer 4, the spatial distribution due to the birefringence ofthe liquid crystal layer 4 which corresponds to the space pattern of theincident light is produced.

The thus-written spatial distribution caused by the birefringence or theoptical rotational power of the liquid crystal 4 is read by causingreading light as polarized rays to enter the liquid crystal layer 4 fromthe right-hand side in FIG. 1 through a polarizer and projecting thelight reflected by the reading light reflecting plate 6 onto a screenthrough the polarizer.

FIG. 2 shows the structure of a liquid crystal display device as aprojection display. The reference numeral 8 represents a liquid crystallight valve and 9 a voltage applying means. As the light writing means,a laser light source such as a CRT, or an He-Ne laser or a semiconductorlaser combined with a two-dimensional scanner 10 is used. The referencenumeral 11 denotes a reading light source, 12 a polarization beamsplitter, and 13 a screen.

The laser light is scanned by the two-dimensional scanner 10 and animage is written into the liquid crystal valve 8. The liquid crystalvalve 8 is irradiated with the light from the reading light source 11which is polarized through the polarization splitter 12 as predeterminedpolarized rays. Since the liquid crystal valve 8 reflects the polarizedrays respectively which are based on the written data, the polarizedrays having a two-dimensional distribution which is determined by thewritten data are separated by the polarization beam splitter 12, and theimage is displayed on the screen 13.

In this liquid crystal display device, the writing light and the readinglight are emitted from different light sources. It is therefore possibleto change the wavelength of a two-dimensional image or change incoherentlight into coherent light by using this liquid crystal display device.Since the liquid crystal display device has a higher spatial resolutionin principle than a light valve using BSO (Bismuth Silicon Oxide), it ispossible to incorporate the liquid crystal display device into alarge-screen high-definition projection system.

A liquid crystal having bistability, which is a component of a liquidcrystal light valve, will now be described briefly.

As a liquid crystal which exhibits bistability, a ferroelectric liquidcrystal is conventionally used. The operation of a ferroelectric liquidcrystal will be described in the following.

In FIGS. 3 and 4, the reference numeral 14 represents a ferroelectricliquid crystal molecule. A long and narrow liquid crystal molecule suchas that shown in FIGS. 3 and 4 exhibits an anisotropic reflective indexbetween the major axial direction and the minor axial direction. Theliquid crystal has spontaneous polarization 15, 16 in the verticaldirection with respect to the major axis of the molecule incorrespondence with the externally applied electric field. Even afterthe electric field is cut, the orientation of the liquid crystal remainsstable. When this liquid crystal is used for a display device, the majoraxes of the liquid crystal molecules are oriented approximately parallelto the substrate. As shown in FIG. 4, the liquid crystal molecules areoriented in a first orientation stable state with respect to an electricfield E15 applied in the direction of the reverse side of the drawing,and in a second orientation stable state 16 with respect to an electricfield applied in the opposite direction to the reverse side of thedrawing.

The change in the orientation due to an electric field is caused onlywhen the energy applied from outside the liquid crystal, namely, theproduct of the electric field intensity (voltage) and the voltageapplying time exceeds a constant critical value (the thresholdproperty). The critical value of the product of the electric fieldintensity (voltage) and the voltage applying time is called a criticalpulse area and hereinunder will be referred to as "CPA". When theorientation state of the liquid crystal is the first orientation stablestate, even if the electric field E16 which does not exceed the CPA isapplied to the liquid crystal in the direction opposite to the reverseside of the drawing during a certain voltage applying time, theorientation state of the liquid crystal does not change. If the appliedvoltage exceeds the CPA (this voltage is defined as a threshold voltage(Vth)), however, the orientation stable state of the liquid crystal isinverted.

As described above, although the photoconductive layer 7 has a highresistance in the dark state (hereinunder referred to as "darkresistance"), when the photoconductive layer 7 is irradiated withwriting light, the resistance of the photoconductive layer 7 at theportion which is irradiated with the writing light is lowered due to thephotoconductive effect, and the voltage applied to the liquid crystalexceeds the threshold (Vth), and the orientation state of the liquidcrystal changes. This operation is shown in time series in FIGS. 5a to5c. The time axes are common to FIGS. 5a to 5c. In FIG. 5a, the symbolVappl represents the waveform of the voltage applied between thetransparent conductive films 2, A a reset pulse and B a writing period.The reference numeral 21 represents a writing voltage pulse, and 22 and23 erasing light and writing light, respectively, projected onto thephotoconductive layer. In FIG. 5b, the symbol V_(LC) represents avoltage applied to the liquid crystal layer, and in FIG. 5c, the symbolIrrl represents the intensity of the reflected reading light at theportion which is irradiated with light.

The space between the opposing electrodes (transparent electrodes) 2 isfirst irradiated with erasing light in synchronism with a reset pulsefor applying -Vth so that the entire liquid crystal layer assumes oneorientation stable state. The liquid crystal layer is then irradiatedwith writing light having a predetermined two-dimensional pattern insynchronism with the writing period. Since a voltage exceeding thethreshold Vth is applied to the liquid crystal layer at the portionwhich is irradiated with the light due to the photoconductive effect,the orientation state changes and the intensity of reading light isaccordingly distributed.

By using a laser light source combined with the two-dimensional scanner10 shown in FIG. 2 as the writing light projecting means, it is possibleto record any given two-dimensional image on a liquid crystal lightvalve. The thus-written image is obtained by projecting the readinglight with which the liquid crystal layer is irradiated through apolarization beam splitter 12 and which is separated by the polarizationbeam splitter 12, optically rotated in the liquid crystal layer 4, andreflected by the reading light reflecting plate 6 onto the screen 13.The optical rotation by the liquid crystal reaches its maximum when theangle 2θ between the first orientation stable state and the secondorientation stable state is 45° .

In order to realize the writing operation by the liquid crystal displaydevice, it is necessary that the resistance of the photoconductive layer7 which is not irradiated with writing light is sufficiently higher thanthat of the liquid crystal layer 4, while the resistance of thephotoconductive layer 7 which is irradiated with writing light is notmore than the resistance of the liquid crystal layer 4. In order torealize the high spatial resolution of the device, it is necessary thatthe film thicknesses of the liquid crystal layer 4 and thephotoconductive layer are small in addition to the condition that thedark resistance of the photoconductive layer is sufficiently high.

In the above-described conventional liquid crystal display device, ahydrogenated amorphous silicon film is used for the photoconductivelayer 7, or CdS, crystalline silicon and BSO are conventionally used.Among these, a hydrogenated amorphous silicon film, which is formed byplasma CVD, is advantageous in that it is comparatively easier to set aresistance value in comparison with any other material, in that theadhesiveness with an underlayer is high and in that the photosensitivityin the visible light range is high.

However, the dark resistance of the hydrogenated amorphous silicon filmformed by plasma CVD (Chemical Vapor Deposition) is 1×10⁹ to 1×10¹¹ Ω·cmin the case of a non-doped film, and at most 1×10¹² Ω·cm in the case ofwhat is called a boron lightly doped film. These resistances cannot besaid to be sufficiently higher than the general resistivity 1×10¹¹ to a×10¹² Ω·cm of a liquid crystal. It is therefore necessary to increasethe dark resistance of the hydrogenated amorphous silicon film by somemeans in the case of using it for the photoconductive layer 7.

For this purpose, a method of making the hydrogenated amorphous siliconfilm much thicker than the liquid crystal layer 4 (e.g., thehydrogenated amorphous silicon film is formed to a thickness of about 10μm while the liquid crystal layer 4 is 2 μm thick), and a method offorming the photoconductive layer 7 from a hydrogenated amorphoussilicon film having a pin structure and executing the writing operationby applying a reverse bias to the pin photodiode are conventionallyadopted. However, if the thickness of the photoconductive film 7 isincreased, the spatial resolution of the liquid crystal display deviceis disadvantageously lowered. In addition, if the photoconductive layer7 has a pin structure, the waveform of the voltage applied to the liquidcrystal layer 4 becomes different from the waveform of the drivingvoltage, so that the liquid crystal deteriorates during periods of longdriving.

The operation temperature of the liquid crystal display device islimited to a definite temperature range. If this temperature range ishigher than room temperature, the dark resistance of the hydrogenatedamorphous silicon film and the activating energy of the reverse currentof the pin photodiode are higher than those of a liquid crystal, and itis thus difficult to make the dark resistance of the photoconductivelayer 7 higher than the resistance of the liquid crystal layer 4.

In the above-described conventional liquid crystal display device, if aCRT is used as an optical writing means, the resolution of the opticalwriting type liquid crystal light valve is limited by the resolution ofthe CRT as the light projecting means. On the other hand, if a laserlight source combined with the two-dimensional scanner shown in FIG. 2is used as the optical writing means, it is possible to increase thespatial information density of the writing light by converging the laserbeam onto the photoconductive layer and using this beam for rasterscanning, so that the optical information recorded with a highresolution which is characteristic of an optical writing type liquidcrystal light valve can be expected.

As described above, the dark resistance of the photoconductive layer 7cannot be said to be sufficiently higher than the resistance of theliquid crystal layer 4.

Therefore, if a writing voltage is applied between the transparentconductive films 2 during the writing period B, a distributed voltagewhich is determined by the resistance between the photoconductive layer7 and the liquid crystal layer 4 is applied to the liquid crystal layer4. Therefore, if the writing period B is long as in the case ofconducting optical writing with high definition by a laser light sourcecombined with the two-dimensional scanner 10, the energy applied to theliquid crystal layer 4 which is not irradiated with light exceeds theCPA, which changes the orientation in the area which is not irradiatedwith light and the intensity of the reflected light at this portiongradually rises disadvantageously, as indicated by the broken line C inFIG. 5c.

FIG. 6 is a timing chart for explaining the procedure for writing datainto a liquid crystal light valve which is applied to a conventionalimage-forming device described in Japanese Patent Laid-Open No. Hei1-241528. In this example, a laser beam is also converged onto a liquidcrystal light valve in order to scan, and an image is written byutilizing the lowering of the resistance of the photoconductive layer 7.In FIG. 6, (a) represents timing for a timing signal generated by asweep starting signal, (b) timing for a driving field for driving aliquid crystal light valve and (c) timing for a video signal formodulating the laser light.

The operation of the conventional liquid crystal display device will beexplained. In writing data into the liquid crystal light valve, rasterscanning is conducted by a laser beam through a polygon mirror scannerand a galvanometer mirror scanner, and a square wave having bothpositive and negative polarities being applied during scanning, asindicated by (b).

The writing operation is carried out at specific

a polarity of the driving voltage. In this example, the video signalindicated by (c) is supplied when the polarity is negative and thewriting operation is conducted. A positive voltage is applied so as tocancel the direct current component of the pulse during writing andwhile the voltage is applied, the writing operation is suspended.

In the above-described conventional method of writing data into theliquid crystal light valve, the writing operation must be suspendedduring the application of a voltage having opposite polarity to that ofthe applied writing voltage.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to eliminate theabove-described problems in the prior art, to provide a liquid crystaldisplay device which is capable of matching the dark resistance of aphotoconductive layer in the dark state with the resistance of theliquid crystal layer, is capable of obtaining a high spatial resolution,and is capable of a stable operation irrespective of the operationtemperature.

It is another object of the present invention to provide a writingapparatus for a liquid crystal light valve which enables a writingoperation irrespective of the polarity of the voltage applied, and whichalso enables partial rewriting.

It is still another object of the present invention to provide ahigh-definition optical writing type liquid crystal light valve whichprevents the inversion of the liquid crystal at the portion which is notirradiated with light during a writing operation.

To achieve this aim, in a first aspect of the present invention there isprovided an optical writing type liquid crystal light valve comprising:

a pair of electrodes; a photoconductive layer which is clamped by thepair of electrodes and the conductivity of which is changed by lightirradiation; and a liquid crystal layer, the molecular alignment ofwhich changes when a voltage is applied. The photoconductive layerincludes an inorganic insulating film formed thereon in such a manner asto face one of the electrodes and a laminate of films formed in such amanner as to face the liquid crystal layer.

According to an optical writing type liquid crystal light valve providedin the first aspect of the present invention, since an inorganicinsulating film is provided between the electrode substrate and thehydrogenated amorphous silicon film of the photoconductive layer and thecarrier injection is suppressed by the action of the inorganicinsulating film, it is possible to increase the dark resistance of thephotoconductive layer, thereby producing a liquid crystal displayapparatus having good displaying properties. The increase in the darkresistance of the photoconductive layer reduces the blurring of an imagecaused by the diffusion of the carriers in the horizontal direction,thereby providing a high spatial resolution. In addition, the increasein the dark resistance enables a stable operation even at a highoperation temperature.

The inorganic insulating film is preferably an SiO₂ film, and thethickness thereof is preferable 20 to 200 nm.

In a second aspect of the present invention, there is provided a writingapparatus for an optical writing type liquid crystal light valve havinga photoconductive layer which is clamped by a pair of electrodes and theconductivity of which changes by light irradiation, and a liquid crystallayer the molecular alignment of which changes in accordance with avoltage applied and which has bistability so as to enable an imagesignal to be written or erased by light irradiation. The writingapparatus comprises.

a laser beam light source for generating at least a writing laser beamfor writing an image signal into the liquid crystal light valve and anerasing laser beam for erasing the contents written into the lightvalve;

a main scanning circuit for driving at least two laser beams to bedeflected in the direction of a scanning line of the liquid crystallight valve so as to write the image signal by the writing laser beamwhile erasing the contents written on the next scanning line into whichan image signal is to be written;

a timing signal generator for generating a main scanning starting signalevery time the scanning of a scanning line by the laser beams is startedby the main scanning circuit;

a polarity change-over switch circuit for inverting the polarity of theimage signal for each scan in correspondence with the generation of themain scanning starting signal by the timing signal generator;

a laser source driving circuit for driving the laser source so as to theemit the writing laser beam in accordance with the image signal with thepolarity inverted by the polarity change-over switch circuit;

a liquid crystal light valve driving circuit for inverting the polarityof the driving voltage applied to the liquid crystal light valve incorrespondence with the generation of the main scanning starting signalby the timing signal generator; and

a sub scanning circuit for driving the erasing laser beam and thewriting laser beam so as to be deflected such that the erasing laserbeam and the writing laser beam scan the next scanning line during eachscanning operation.

According to the writing apparatus provided in the second aspect of thepresent invention, since a writing operation is carried out while thenext scanning line is being erased at both polarities of a drivingvoltage of the liquid crystal light valve, it is possible to realizehigh-speed laser beam drawing while maintaining the threshold voltage ofthe liquid crystal and to use the applied voltage of both polaritieswithout waste. Partial writing is also possible.

In a third aspect of the present invention, there is provided a writingapparatus for an optical writing type liquid crystal light valve havinga pair of electrodes, a photoconductive layer which is clamped by a pairof electrodes and the conductivity of which is changed by lightirradiation, and a liquid crystal layer having bistability so as toenable an image signal to be written or erased by light irradiation, thewriting apparatus comprising:

a voltage applying circuit for applying a voltage between the pair ofelectrodes;

a light projecting device for projecting rays which correspond to animage signal for one frame to the photoconductive layer bytwo-dimensional scanning and writing the image signal into the liquidcrystal layer by lowering the conductivity of the photoconductive layerat the portion which is irradiated with light so as to increase thevoltage at that portion which is applied to the liquid crystal layer;and

a controller for controlling the voltage applying circuit so as tosuspend the application of the voltage between the pair of electrodes atleast in a predetermined period during the operation of writing theimage signal for one frame by the light projecting device.

According to the writing apparatus for an optical writing type liquidcrystal light valve provided in the third aspect of the presentinvention, the inversion of the liquid crystal layer at the portionwhich is not irradiated with light caused by the voltage distributed tothe liquid crystal layer due to the photoconductive layer having a lowerdark resistance than the liquid crystal layer, is suppressed. This meansthat optical information recording by an easy method is made possiblewithout impairing the high-definition writing property which ischaracteristic of an optical writing type liquid crystal light valve.

The period during which the application of a voltage is suspended ispreferably in the range of 10 to 40% of the writing time. The lightprojecting means projects rays so as to scan a plurality of scanninglines. The number of times the application of a voltage is suspended ispreferably 0.5 to 2 times the number of scanning lines for one frame.

The above and other objects, features and advantages of the presentinvention will become clear from the following description of thepreferred embodiments thereof, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a conventional liquid crystal displayapparatus;

FIG. 2 shows the structure of a display apparatus using an opticalwriting type liquid crystal light valve;

FIG. 3 is an explanatory view of the operation of a ferroelectric liquidcrystal;

FIG. 4 is a partial enlargement of the explanatory view shown in FIG. 3;

FIGS. 5a to 5c show a change of the voltage and the intensity of readinglight with time for explaining a conventional method of driving anoptical writing type liquid crystal light valve;

FIG. 6 is a timing chart of the procedure for writing data into aconventional liquid crystal light valve;

FIG. 7 is a sectional view of a first embodiment of a liquid crystaldisplay apparatus according to the present invention;

FIG. 8 is a sectional view of a second embodiment of a liquid crystaldisplay apparatus according to the present invention;

FIG. 9 is a block diagram of an embodiment of a writing apparatus for aliquid crystal light valve according to the present invention;

FIG. 10 is an enlarged sectional view of the structure of the liquidcrystal light valve for the embodiment shown in FIG. 9;

FIGS. 11A and 11B are explanatory views of the liquid crystal lightvalve shown in FIG. 10 in the state of data being written thereinto;

FIGS. 12(a)-12(c) are timing charts of a driving signal for a laserlight source in the embodiment shown in FIG. 9; and

FIGS. 13A to 13C show a change of the voltage and the intensity ofreading light with time for explaining the waveform of a driving signalfor another embodiment of an optical writing type liquid crystal lightvalve according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be explained with reference tothe accompanying drawings. FIG. 7 is a sectional view of an embodimentof a liquid crystal display apparatus according to the presentinvention. In FIG. 7, the reference numeral 1 represents an opposingpair of glass substrates and 2 a transparent conductive film formed onthe inner surface of the glass substrate 2. The glass sub strates 1 andthe transparent conductive films 2 in combination constitute anelectrode substrate. The reference numeral 5 denotes an orientation filmformed on one of the transparent conductive films 2, and 4 a liquidcrystal layer formed on the orientation film 5. A reading lightreflecting film 6 is formed on the liquid crystal layer 4 through theorientation film 5. An inorganic insulating film 32 composed of asilicon oxide film is formed on the other transparent conductive film 2and a hydrogenated amorphous silicon film 31 is formed on the inorganicinsulating film 32. The inorganic insulating film 32 and hydrogenatedamorphous silicon film 31 constitute a photoconductive layer 33. Theother surface of the hydrogenated amorphous silicon film 31 isintegrally provided with the reading light reflecting film 6.

In manufacturing the liquid crystal display apparatus having theabove-described structure, after an ITO film was formed on the glasssubstrate 1 as the transparent conductive film 2, the inorganicinsulating film 32 composed of SiO₂ was formed on one of the transparentconductive film 2 by sputtering, except for at the electrode terminalleading portions. The thickness of the inorganic insulating film 32 was20 to 200 nm. The hydrogenated amorphous silicon film 31 was then formedon the inorganic insulating film 32 by using silane gas and diborane gasby plasma CVD to a thickness of 3 μm. A liquid crystal cell composed ofthe orientation film 5, the liquid crystal layer 4 and the reading lightreflecting film 6 was formed between the hydrogenated amorphous siliconfilm 31 and the other transparent electrode film 2. For the liquidcrystal layer 4, a ferroelectric liquid crystal CS-1014 (produced byChisso Corporation) was used. An obliquely deposited SiO film was usedas the orientation film 5. The cell gap of the liquid crystal was 2 μm.The temperature of the SmC* phase of the liquid crystal at which theliquid crystal is used for displaying operation was 40° to 50° C. Amulti-layer dielectric film formed by deposition was used as the readinglight reflecting film 6.

The operation temperature of the liquid crystal display apparatus wasmaintained at 45°. The resistivity of the liquid crystal at thisoperation temperature was about 3.3×10¹¹ Ω·cm and the resistivity of thehydrogenated amorphous silicon film 31 in a dark state was about 1×10¹¹Ω·cm.

The operation of this embodiment will now be explained. The mainoperations for writing and reading are the same as in the related art.When a bias is applied between the transparent conductive films 2 in adark state, since the carrier injection from the transparent conductivefilms 2 to the hydrogenated amorphous silicon film 31 is suppressed byvirtue of the inorganic insulating film 32 which is clamped between thehydrogenated amorphous silicon film 31 and the transparent conductivefilm 2, the effective dark resistance of the photoconductive layer 32increases in comparison with the structure of a conventional apparatus.The resistance value of the photoconductive layer 33 in a dark state canbe increased or decreased by varying the thickness of the inorganicinsulating film 32. The photoconductive layer 33 is therefore capable ofsufficiently coping with the operation at a high temperature, which isconventionally difficult due to the lowering of the resistance of thehydrogenated amorphous silicon film 31.

Under irradiation of the writing light, if a voltage applied to thetransparent conductive film and the thickness of the inorganicinsulating film 32 are appropriately selected, a breakdown is producedin the inorganic insulating film 32 due to the lowering of theresistance of the hydrogenated amorphous silicon film 31. This breakdownleads to the lowering of the resistance of the entire photoconductivelayer 33, and, as a result, a bias is applied to the liquid crystallayer 4.

In order to make the merits of the above-described liquid crystaldisplay apparatus clear, samples of a liquid crystal displayingapparatus which were provided with the inorganic insulating films 32having various thicknesses were prepared One of which was not providedwith an these samples were compared with each other. The results areshown in Table 1.

                  TABLE 1                                                         ______________________________________                                                Sample No.                                                                    1       2      3        4    5                                        ______________________________________                                        Thickness  0        10     30     100  300                                    of                                                                            SiO.sub.2 film                                                                Displaying                                                                    properties                                                                    Spatial   30        30     60     100   30                                    resolution                                                                    (1p/mm)                                                                       Writable  10 to     10 to  13 to  19 to                                                                              26 to                                  voltage   2         12     20     30   40                                     region                                                                        (V)                                                                           ______________________________________                                    

Data was written into the liquid crystal display apparatus by applying arectangular voltage pulse of 100 nsec duration in the state in whichparallel He-Ne laser rays were caused to enter the photoconductive layer33 through a mask pattern for evaluating resolution. The intensity ofthe writing light was 10 μW/cm². In Table 1, the maximum value of thespatial resolution when the voltage of the rectangular voltage pulse forwriting was varied and the voltage region which allowed writing areshown as the displaying properties.

By clamping the inorganic insulating film 32 of a silicon oxide filmbetween the transparent conductive film 2 and the hydrogenated amorphoussilicon film 31, the resolution of the liquid crystal display apparatuswas improved to about 100 lp/mm. This is considered to be because theblurring of the image caused by the diffusion of carriers in thehorizontal direction was reduced due to the increase in the effectivedark resistance of the photoconductive layer 33. In addition, since thevoltage required for writing was increased and, further, the voltagerange which allows writing was increased with the increase in thethickness of the inorganic insulating film 32, a stable writingoperation was enabled. These advantages are prominent when the thicknessof the inorganic insulating film 32 is not less than 20 nm.

However, if the thickness of the inorganic insulating film 32 exceeds200 nm, since the lowering of the resistance of the photoconductivelayer 33 is reduced, it is necessary to greatly increase the voltageapplied or the quantity of light for writing. In addition, due to thelowering of the capacitance of the photoconductive layer 33, theblurring of the image increases. For these reasons, it is obvious thatthe appropriate thickness of the inorganic insulating film 32 is 20 to200 nm.

FIG. 8 shows the structure of a second embodiment of a liquid crystaldisplay apparatus according to the present invention. In thisembodiment, the inorganic insulating films 32 composed of a siliconoxide film are formed on both sides of the hydrogenated amorphoussilicon film 31, thereby constituting a photoconductive layer 34. Theother structures are the same as in the first embodiment.

In the second embodiment, one of the inorganic insulating films 32 isclamped between the transparent conductive film 2 and the hydrogenatedamorphous silicon film 31 an increases the dark resistance of thephotoconductive layer 34. Since the photoconductive layer 34 exhibitsapproximately the same current/voltage characteristic irrespective ofthe polarity of the voltage applied thereto, it is possible to preventthe deterioration of the liquid crystal which is conventionally causedwhen a pin photodiode is used for the photoconductive layer 34.

In both of the above-described embodiments, the liquid crystal displayapparatus is what is called a reflection type liquid crystal displayapparatus which has the reading light reflecting film 6. The readinglight reflecting film 6 is provided in order to increase the efficiencyof the reading light and to separate the photoconductive layer 33 (34)from the optical reading system. It is therefore possible to use theliquid crystal display apparatus of the present invention as atransmission type liquid crystal display apparatus by omitting thereading light reflecting film 6 and using reading light having awavelength which is out of the sensitivity range of the photoconductivelayer 33 (34). In this case, it is also possible to improve thedisplaying properties by forming the photoconductive layer 33 (34)having the above-described structure. Although a ferroelectric liquidcrystal is used for the liquid crystal layer 4, it is possible to usetwist nematic liquid crystal, guest-host type liquid crystal or liquidcrystal dispersion type polymer instead. In addition, although an SiO₂film is used as the inorganic insulating film 32 in these embodiments,an SiN or SiON film may be used instead.

An embodiment of a writing apparatus for a liquid crystal light valveaccording to the present invention will be explained hereinunder. FIG. 9is a block diagram of the structure of this embodiment. In FIG. 9, thereference numeral 101 represents a laser light source for generating twospot beams, 102 a main scanning device for deflecting the laser beamwhich is emitted from the laser light source 101 in the horizontaldirection, 103 a sub scanning device for deflecting the laser beam inthe vertical direction, 104 an optical scanning system for convergingthe beam deflected by the main scanning device 102 and the sub scanningdevice 103 for the scanning operation, and 105 a liquid crystal lightvalve for storing the portion which is irradiated with light.

The reference numeral 106 denotes an image signal generator. The imagesignal generated by the image signal generator 106 is inputted to apolarity change-over switch circuit 107, which switches the image signalevery scanning operation in accordance with the timing signal suppliedfrom a timing signal generator 110. In other words, when black portionsare written on a white background during one scanning operation, whiteportions are written on a black background in the next.

The image signal with the polarity switched by the polarity change-overswitch circuit 107 is inputted to a laser driving circuit 108 whichdrives the laser light source 101.

A liquid crystal light valve driving circuit 109 applies a square wavehaving both polarities to the liquid crystal light valve 105 inaccordance with the timing signal 110 supplied from the timing signalgenerator 110. The timing signal generator 110 detects the initiation ofhorizontal sweeping by the main scanning device 102 and outputs a timingsignal to the polarity change-over switch circuit 107 and the liquidcrystal light valve driving circuit 109, as described above.

A light source 11 generates projecting rays for displaying the image onthe liquid crystal light valve 105. The rays are projected onto theliquid crystal light valve 105 through an optical projection system 112so as to read out the image on the liquid crystal light valve 105 anddisplay the image on a screen 11.3

As the liquid crystal light valve 105, those provided with an inorganicinsulating film such as those shown in FIGS. 7 and 8 are preferable, buta light valve such as that shown in FIG. 10 may also be used. FIG. 10 isan enlarged sectional view of a liquid crystal light valve. On theinside surfaces of an opposing pair of glass substrates 201a, 201b,transparent electrodes 202a, 202b are formed respectively. Aphotoconductor 207 and a multi-layer dielectric film mirror 206 areconnected to the transparent electrode 202b. A liquid crystal 205 isclamped between the multi-layer dielectric film mirror 206 and thetransparent electrode 202a through liquid crystal orientation films 203band 203a, respectively. The upper and lower ends of the liquid crystal205 are held by spacers 204a and 204b, respectively.

The operation of the liquid crystal light valve shown in FIG. 10 willnow be explained. In this embodiment, a liquid crystal havingbistability such as a ferroelectric liquid crystal is used. Thephotoconductor 207 has a high resistance if it is not irradiated withlight, but the resistance is lowered by light irradiation. When a partof the photoconductor 207 is irradiated with laser light 208 while avoltage is applied between the transparent electrodes 202a and 202b, theelectric field applied to the liquid crystal 205 increases with thelowering of the resistance of the photoconductor 207 at the portionwhich is irradiated with light, and the liquid crystal is only driven atthe portion which is irradiated with light.

In this embodiment, since the liquid crystal 205 has bistability withrespect to an electric field, the liquid crystal driven above thethreshold maintains the writing state even when the voltage appliedthereto lowers after light irradiation.

Consequently, it is possible to record image information on the liquidcrystal by raster sweeping the laser light 208 and to display the imageon a screen by projecting rays 209. The ferroelectric liquid crystal hasbistability, as described above, and image information is recorded by abinary system representing brightness and darkness.

The stable state of the liquid crystal and the way of corresponding tobrightness and darkness for display are different depending upon thestructure of the optical projection system 112, but it is here assumedfor the purpose of simplifying the explanation that when a positivevoltage is applied to the transparent electrode 202b on thephotoconductor 207 side, brightness is displayed, while when a negativevoltage is applied thereto, darkness is displayed.

FIGS. 11A and 11B show the liquid crystal light valve 105 in thisembodiment in the state of data being written thereinto.

In FIG. 11A, data are written on the n-th scanning line, and in FIG.11B, data are written on the (n+1)th scanning line. The hatched portionsin FIGS. 11A and 11B denote laser spot beams. A writing spot beam 301aand an erasing spot beam 302a are moved from the left to the right inthe drawing by the main scanning means 102.

In FIG. 11A, a negative voltage is applied to the liquid crystal lightvalve 105 by the liquid crystal light valve driving circuit 109, and theportion which is irradiated with light is recorded in a dark state. Thewriting spot beam 301a is projected onto the portions which correspondto the black portions of the image signal so as to record the blackpixels on a white background.

The erasing spot beam 302a is moved onto the (n+1)th scanning line whilelight is projected thereto so as to change the entire portion of the(n+1)th scanning line to black.

In the next scanning operation, a writing spot beam 301b is moved on the(n+1) th scanning line by the sub scanning means 103. At this time, theliquid crystal light valve driving circuit 109 inverts the polarity ofthe voltage applied in accordance with the sweep start signal suppliedfrom the timing signal generator 110, and a positive voltage is appliedto the liquid crystal light valve 105.

Since the entire part of the (n+1)th scanning line has been changed intoblack by the erasing spot beam 302a, as described above, the portionsirradiated with a writing spot beam 301b are recorded as white portionson the black background.

In this way, since the correspondence of light irradiation to brightnessand darkness in FIG. 11B is opposite to that in FIG. 11A, the signalsupplied from the image signal generator 106 is supplied to the laserdriving circuit 108 after the polarity is inverted by the polaritychange-over switch circuit 107, thereby enabling light irradiation atthe white portions of the image signal.

FIG. 12 shows the state of the driving signal. In FIG. 12, a liquidcrystal light valve driving voltage (a), an image signal (b) and a laserdriving signal are shown with the time axis as the abscissa.

The image signal (b) has a periodical waveform consisting of one pixelfor white and two pixels for black, and when the liquid crystal lightvalve driving voltage (a) is negative, the laser driving signal (c) forthe black portions of the image signal is turned on, while when theliquid crystal light valve driving voltage (a) is positive, the laserdriving signal (c) for the white portions of the image signal is turnedon.

In FIG. 22B, the erasing spot beam 302b constantly projects light as inFIG. 11A so as to change the entire portion of the (n+2)th scanning lineto white. By repeating the operation shown in FIGS. 11A and 11B withrespect to all scanning lines, image rewriting which simultaneouslyexecutes erasure and writing is realized.

In the above-described embodiment, it is possible to adopt a differentcombination of polarities of the image signal, the liquid crystal lightvalve driving voltage and the laser driving voltage depending upon thestructure of the optical projection system. In this case, thefundamental operation is the same.

Although only one erasing spot beam and one writing spot beam are usedin this embodiment, a plurality of them may be used. In that case, it ispossible to write data on the same number of scanning lines as thenumber of spot beams in one main scanning operation, and interlacedscanning corresponding to the number of spot beams is necessary in thesub scanning.

In addition, since erasure and writing are only executed at the portionwhich is irradiated with laser spot beams, if only a part of a scanningline is scanned, partial rewriting is also possible.

Another embodiment of a writing apparatus for a liquid crystal lightvalve according to the present invention will be explained hereinunder.The operation of this embodiment is shown in time series in FIGS. 13A to13C. The time axes are common to FIGS. 13A to 13C. In FIG. 13A, thesymbol Vappl represents the waveform of the voltage applied between theelectrodes, and the reference numeral 401 represents a reset pulse, 402a writing period, 403 a no voltage applying period, namely, the periodin which no voltage is applied during the writing period, and 404projected writing light. In FIG. 13B, the symbol V_(LC) represents avoltage applied to the liquid crystal layer, and in FIG. 13C, the symbolIrrl represents the intensity of the reflected reading light.

The liquid crystal light valve used for this embodiment may have thesame structure as those shown in FIGS. 7 and 8, but the light valveshown in FIG. 10 is also usable. An embodiment used for the liquidcrystal light valve shown in FIG. 10 will be explained here. In thisembodiment, an obliquely deposited SiO film was used as the liquidcrystal orientation films 203a and 203b. For the liquid crystal layer205, a ferroelectric liquid crystal CS-1014 (produced by ChissoCorporation) was used. The multi-layer dielectric film mirror 206 wasproduced by laminating a multiplicity of sheets of dielectric film bydeposition. As the photoconductor 207, an amorphous silicon film formedby plasma CVD was used. The cell gap of the liquid crystal 205 was setto be 2 um by using spacers 204a and 204b. As the writing laser beam208, a semiconductor laser having a wavelength of 670 nm was used.

The operation of this embodiment will now be explained. In FIGS. 13A to13C, a pulse having a sufficiently high voltage (e.g., 20 V) and asufficiently large time width (e.g., 0.8 msec) for making the entireportion of the liquid crystal layer a dark state even if it is notirradiated with light, namely the reset pulse 401, was applied betweenthe electrodes. The other fundamental operation was the same as in therelated art shown in FIGS. 5a to 5c. In writing, an optical pulse havinga width of 100 ns was projected for each dot for two-dimensionalscanning. After 1500 writing pulses were written in the main scanningdirection, no voltage was applied for a period of 100 μsec. The samewriting operation was repeated on the next scanning line. When a voltagenaving the waveform shown in FIGS. 5a to 5c was applied, the orientationof the liquid crystal was inverted even at the portion which was notirradiated with PG,37 light. In contrast, in this embodiment, by settinga no voltage applying period during the process of optical scanning, theorientation of the liquid crystal layer was not inverted at the portionwhich was not irradiated with light. In this embodiment, immediatelyafter the writing period 402 for one frame, the next reset pulse 401 wasapplied, but a no voltage applying period may be set between theseperiods in order to maintain the image.

Although 1500 writing pulses were applied before the no voltage applyingperiod in this embodiment, the number of writing pulses applied beforethe no voltage applying period may fundamentally be determined asanywhere in the range which does not cause the inversion of the liquidcrystal at the portion which is not irradiated with light. It istherefore possible to set a no voltage applying period after theapplication of each writing pulse, but it is preferable from the pointof view of practicality such as the rewriting application of not lessthan 500 writing pulses. On the other hand, if it is assumed that thetime obtained by dividing the CPA by the voltage applied to the liquidcrystal at the portion which is not irradiated with light is thethreshold time of the portion which is not irradiated with light, andthat the value obtained by dividing the threshold time by the width ofthe writing pulse is a critical number of writing pulses, since theliquid crystal at the portion which is not irradiated with light invertsif the number of writing pulses exceeds the critical number of writingpulses, it is preferable that the number of writing pulses is not morethan 90% of the critical number of writing pulses.

Although 100 usec is set as the no voltage applying period, the novoltage applying period is not restricted thereto. Since the existenceof the no voltage applying time sufficiently separates the writingvoltage pulses and shortens the writing time, it may take any value inthe range of 50 μsec to 300 μsec.

In this embodiment, the no voltage applying period is set on every line,but it may be set in the process of writing data on only one line, or onevery few lines.

A liquid crystal light valve utilizing the reflection of reading lightby the multi-layer dielectric film mirror 206 is described in thisembodiment, but the present invention is also applicable to atransmission type liquid crystal light valve which is not provided witha reading light reflecting film.

Although the liquid crystal was initialized to an orientation stablestate of an entirely dark state and optical recording (positiveformation) was carried out by changing the dark state into a brightstate in this embodiment, the liquid crystal may be initialized to anorientation stable state of an entirely bright state and optical writing(negative formation) may be carried out by changing the bright stateinto a dark state. Alternatively, optical writing both in a dark stateand in a bright state without initialization is also possible.

In this embodiment, the photoconductor 207 consists of an amorphoussilicon film, but the photoconductor 207 may have a pin structure or itmay be a laminate of an amorphous silicon film and an inorganic oxidefilm provided between the amorphous silicon film and the transparentelectrode. In the case of adopting a pin structure, erasure (orientationto a dark state) is carried out by applying a forward voltage whilewriting is carried out by applying an inverse voltage.

Even in the embodiments shown in FIGS. 9 to 12 in which the polarity isinverted on every horizontal line, charges are sometimes stored. In thiscase, it is preferable to set the no voltage applying period in theprocess of writing data on one line.

While there has been described what are at present considered to bepreferred embodiments of the invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claims cover all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:
 1. A writing apparatus for an optical writing typeliquid crystal light valve having a photoconductive layer which isclamped by a pair of electrodes, the conductivity of which is changed bylight irradiation, and a liquid crystal layer the molecular alignment ofwhich changes by an applied voltage and which has bistability so as toenable an image signal to be written or erased by light irradiation, thewriting apparatus comprising:a laser beam light source for generating atleast a writing laser beam for writing an image signal into the liquidcrystal light valve and an erasing laser beam for erasing the contentswritten into the light valve; a main scanning means for driving at leastthe two laser beams to deflection scanning in the direction of ascanning line of the liquid crystal light valve so as to write the imagesignal by the writing laser beam while erasing the contents written onthe next scanning line into which an image signal is to be written; atiming signal generator for generating a main scanning starting signalevery time the scanning of a scanning line by the laser beams is startedby the main scanning means; a polarity change-over switch circuit forinverting the polarity of the image signal for every scanning operationin correspondence with the generation of the main scanning startingsignal by the timing signal generator; a laser source driving circuitfor driving the laser source so as to the emit the writing laser beam inaccordance with the image signal with the polarity inverted by thepolarity change-over switch circuit; a liquid crystal light valvedriving circuit for inverting the polarity of the driving voltageapplied to the liquid crystal light valve in correspondence with thegeneration of the main scanning starting signal by the timing signalgenerator; and a sub scanning means for driving the erasing laser beamand the writing laser beam to deflection scanning so that the erasinglaser beam and the writing laser beam scan the next scanning line afterthe completion of each scanning operation.
 2. A writing apparatusaccording to claim 1, wherein the molecular alignment of the liquidcrystal layer changes in accordance with a voltage applied and thephotoconductive layer includes an inorganic insulating film formedthereon in such a manner as to face one of the electrodes and a laminateof amorphous Si films formed in such a manner as to face the liquidcrystal layer.
 3. A writing apparatus for an optical writing type liquidcrystal light valve having a pair of electrodes, a photoconductive layerwhich is clamped by a pair of electrodes, the conductivity of which ischanged by light irradiation, and a liquid crystal layer havingbistability so as to enable an image signal to be written or erased bylight irradiation, the writing apparatus comprising:a voltage applyingmeans for applying a voltage between the pair of electrodes; a lightprojecting means for projecting rays which correspond to an image signalfor one frame to the photoconductive layer by two-dimensional scanningand writing the image signal into the liquid crystal layer by loweringthe conductivity of the photoconductive layer at the portion which isirradiated with light so as to increase the voltage at that portionwhich is applied to the liquid crystal layer; and a controller forcontrolling the voltage applying means so as to suspend the applicationof the voltage between the pair of electrodes at least in apredetermined period during the operation of writing the image signalfor one frame by the light projecting means.
 4. A writing apparatusaccording to claim 3, wherein the predetermined period is in the rangeof 10 to 40% of the writing time.
 5. A writing apparatus according toclaim 4, wherein the light projecting means scans a plurality ofscanning lines by rays and the number of times the application of avoltage is suspended is 0.5 to 2 times the number of scanning lines forone frame.